The research aims to establish the molecular basis of calcium absorption across the small intestine and the mechanism of regulation by vitamin D, dietary calcium, growth and aging, pregnancy and other factors. The absorption involves an active cation pump for calcium and comprises two or more steps, including entry at the mucosal surface and energy-coupled exit at the serosal face. Vitamin D maintains tissue components required for the transport. The identification and characterization on these is the main focus of this proposal. Prior studies have defined a soluble calcium-binding protein. Recently we have identified and partially characterized a vitamin D-dependent particulate, calcium-binding activity of high molecular weight which correlates with the calcium transport and is believed to be a membrane component of the translocation mechanism.